1. Field of the Invention
The present invention relates generally to high-speed digital data transmission systems.
2. Description of the Related Art
Multi-gigabit per second (Gbps) communication between various chips on a circuit board or modules on a backplane has been in use for quite a while. Data transmission is usually from a transmitter that serializes parallel data for transmission over a communication media, such as twisted pair conductors as a cable or embedded in a backplane, fiber optic cable, or coaxial cable(s), to a receiver that recovers the transmitted data and deserializes the data into parallel form. However, data transmission greater than 8 Gbps over communication paths has been difficult to achieve because various signal impairments, such as intersymbol interference (ISI), crosstalk, echo, and other noise, can corrupt the received data signal to such an extent that a receiver is unable to recover the transmitted data at the desired high data rate with an acceptable level of error performance.
Various techniques are employed to improve the performance of the receiver. One technique is to provide the receiver with an analog front end (AFE), having a variable gain amplifier to assure signal linearity within a desired dynamic range and a multi-band adjustable analog (linear) equalizer (AEQ) to compensate for frequency-dependent losses, and an adaptive decision feedback equalizer to compensate for interference and other non-linear distortions of the channel. Even though the quality (e.g., the amount of “eye opening”) of the received signal can be improved by the AEQ, the complexity of the AEQ needed to handle different serial communication protocols (e.g., PCIe Gen3, 12 G SAS, 16GFC, and 10GBASE-KR, all of which are included herein by reference in their entirety) over communication channels ranging from short, highly reflective channels to long-span channels with a poor insertion loss-to-crosstalk ratio (ICR), may be too complicated to implement cost effectively. Further, the amount of frequency-dependent distortion and interference may exceed the capability of the AEQ such that it cannot fully correct for them, resulting in unacceptably poor performance.
One way to improve the quality of the received signal is for the signal transmitter, coupled to the receiver, to drive the channel with signals that have been pre-distorted by a filter. One such filter used to pre-distort the transmitted signal is a finite-impulse response (FIR) filter with adjustable coefficients or tap weights, referred to herein as a TXFIR filter. For lower speed applications, the filter tap weights might be predetermined, i.e., selected from a set of preset filter tap weights, based on the design of the channel and the protocol being implemented. However, with the need for high-speed (e.g., 8 Gbps and above) applications, using a fixed set of TXFIR filter tap weights has not worked well for all transmitter/channel/receiver implementations. Even similar implementations may require significantly different TXFIR filter tap weights values for proper operation due to chip-to- chip electrical parameter variations of the integrated circuits embodying the transmitter and the receiver and the electrical characteristics of the channel media as well.
The standards bodies that administer the various serial communication protocols mentioned above recognized the shortcomings of using fixed TXFIR filter tap weights and provided in the protocols a feedback mechanism utilizing a back-channel to allow for adjustment of the TXFIR filter tap weights during initialization of the transmitter and receiver. The protocols allow for the receiver to adapt the TXFIR filter tap weights by receiving a known data pattern from the transmitter and communicating new filter tap weights values to the transmitter via the back-channel.
Techniques used to adapt the TXFIR filter tap weights include gradient-based approaches, such as the widely used least-mean-squared (LMS) algorithm, that generally rely on the AFE being essentially distortion-free and invulnerable to large signal compression and PVT variations, none of which is possible in existing small geometry implementation technology, e.g., 28 nm and smaller. Moreover, the gradient-based approaches to TXFIR adaptation do not reliably adapt the TXFIR post-cursor tap weight with channel media loss and transmitter launch amplitude variations experienced in typical systems. It has been found that under certain circumstances, various coefficients, such as the coefficient used to set the level of peaking provided by AEQ, saturate at one extreme or another so that the adaptation does not converge, resulting in a failure of communication from the transmitter to the Receiver.
With decreasing process technology geometry and decreasing power supply requirements, transmitter and receiver design for high-speed application is challenging due to increased process-voltage-temperature (PVT) related data path gain variation and power supply headroom-induced signal distortion. The effect of PVT variation is more pronounced at 28 nm and smaller technology. As an example, with a fast process corner at low supply voltage and high operating temperature associated with low transmitter launch on a high loss channel, the AFE might not offer enough signal gain and high frequency signal amplification for error-free communication. Moreover, disproportionate gain variation between a low frequency signal path and a high frequency signal path results in loss of signal fidelity by altering signal linearity. An example of such behavior may be found in a typical compression curve of an exemplary AFE consisting of a low frequency signal path and a high frequency signal path. In a small geometry implementation, it is typical that a DC input signal might suffer 30% attenuation in a low amplitude linear region of the AFE (e.g., 100 mV input) and have a 5 dB compression at large amplitude (e.g., 300 mV input).
On the other hand, with a slow process corner and high supply voltage at low operating temperature associated with a high transmitter launch voltage over a low loss channel, the AFE might exhibit excessive signal compression at higher signal amplitudes and no compression at lower signal amplitudes. This in turn results in loss of signal fidelity by selective signal compression on low frequency and high frequency components of an incoming signal at the receiver. As a result, the conventional data sample phase-aligned error latch-based adaptation algorithms do not develop the correct information for accurate adaptation of receiver coefficients as well as the TXFIR filter tap weights.
Therefore, it is desirable to provide a receiver that can quickly, robustly, and reliably adjust the TXFIR filter tap weights where AFE gain/boost variations and signal linearity challenges resulting from PVT variations exist along with lower power supply-induced signal headroom limitations, thereby improving the overall reliability of communication between the transmitter and the receiver.